This project is concerned with the role of the epididymis in posttesticular sperm maturation. Specifically the morphology of the epididymis in a variety of mammals has been carefully studied with the light and electron microscopes in an attempt to correlate known functions of the duct with its morphology. Physiological and biochemical studies have been conducted on steroid synthesis and metabolism by the epididymis and by its isolated epithelium. At present studies involve the interaction of epididymal sperm with the fluid environment in which they live. Particular attention is being paid to sperm-steroid binding and its possible physiological consequences.